Actuators for producing a mechanical displacement of a member to be driven are employed throughout industry in a wide variety of applications. These include machinery control mechanisms, including valves and linkages, robotics, prosthetics, camera optics, pumps, brakes and power tools to name but a few. The displacement required may be rotary, linear or other translational and of short or long stroke. It may be unidirectional, with a separate return mechanism such as a spring or bidirectional, including reciprocation. The choice of actuator for a particular application often depends on the environment in which it is to be used.
Many forms of actuator for producing linear or other translational displacement of a driven object are known in the prior art. These include straightforward pneumatic and hydraulic piston arrangements and more recently developed devices known as “air muscles” in which inflation of a bladder causes contraction of an outer metal sheath in a manner similar to living muscle contraction. Other forms of linear actuator are electromagnetic, such as the solenoid and the voice coil motor. Such devices have limited extension capabilities.
Electric motors, such as stepper or servo motors, are also convenient drivers for actuator devices but to produce linear displacements their rotary output must be transformed into a linear motion by a suitable conversion mechanism. Many such mechanisms have been employed for this purpose such as the rack and pinion mechanism and the lead screw. In the latter case, a short threaded nut is translated along a long threaded shaft rotated by the motor and is coupled to a member to be driven, such as a print carriage. By appropriate choice of thread pitch or use of additional gearing, the mechanical advantage of this type of mechanism can be increased to produce relatively large extensions for small rotations.
Cam shafts and followers, biased by a return spring, are also widely used, especially in conventional engines, for producing reciprocating linear motion and similar cam follower and spring arrangements are also used in power tools to produce a reciprocating action from a conventional electric motor drive shaft.
There is still scope however for a simple rotary to translational motion conversion mechanism, capable of producing large extensions for a limited angle of rotation and robust enough to be tolerant of hostile environments. The present invention offers such a mechanism.
Also known in the prior art are adjustable shims or spacer arrangements for producing a desired static linear displacement by relative rotation of complementarily shaped discoidal wedges or cams. Such arrangements are described in U.S. Pat. No. 4,433,879 (J. C. Morris) for an “Adjustable Extension-Cam Shim” and in GB published patent application 2331568 (A. Szmidla) for “Wedges and arrangements thereof”.